The present invention relates to an optical transmission system using an IM (Intensity Modulation) method, which is one type of optical fiber communication methods.
Recently in the field of optical fiber communication, increase in signal transmission speed and increase in transmission distance have been rapidly developed. One of the most important factors restricting the transmission distance in such a superhigh speed/long distance transmission is "dispersion" due to optical fiber. Dispersion is a phenomenon, by which lights having different wavelengths are transmitted with different speeds in the optical fiber. Optical spectre of light signals modulated with a high speed contains components having different wavelengths and these components having different wavelengths reach a receiving end at different points of time under influences of the dispersion. It is known that, as a result, large waveform distortion takes place in light waveform after the transmission. A method called dispersion compensation is conceived as a method for avoiding such influences of dispersion. This is a method for preventing waveform distortion after the transmission, by which mediums (optical fiber or grating) having dispersion characteristics opposite to those of the transmission line are inserted into the course thereof so that the mean dispersion of the transmission line is approximately zero.
On the other hand, nonlinear effects of the optical fiber are known as another factor restricting the transmission distance for optical fiber communication. In particular, in the optical transmission by the intensity modulation method, self phase modulation (hereinbelow abbreviated to SPM) effect, which is one of the nonlinear effects of the optical fiber, gives rise to a serious problem. SPM is a phenomenon, by which a refractive index of the optical fiber varies proportionally to variations in the intensity of optical signals, and as a result, extra phase modulation, i.e., frequency chirp (variations in light frequency), is superposed on the light signals in the optical fiber. Light signals having such frequency chirp give rise to important waveform variations after the transmission because of influences of the dispersion, which the optical fiber has. Such influences of SPM is reported in 1 "Nonlinear Fiber Optics", Academic Press, 1992, (ISBN0 -12-045140-9), 2 Technical Reports of Institute of Electronics, Information and Communication Engineers (Japan) OCS92-52, by Kikuchi, et al., Electronic Communication Information Society (1992), etc.
Heretofore, several examples in which the dispersion compensation is effected have been reported also in cases where influences of SPM are great. However the dispersion compensation has been effected therein merely so that the dispersion of the whole transmission line is zero just as in the prior art techniques. Further influences, which the SPM has on the dispersion compensated transmission are almost not studied.